- #1

mattstjean

- 14

- 0

Hi, I am also having trouble with the hockey puck question.

A hockey puck rebounds from a board as shown in my diagram. The puck is in contact with the board for 2.5 ms. Determine avg acceleration of the puck over the interval.

V

I tried the cosine law but I keep getting 44 m/s and not 18. I don't understand how you guys got 18 m/s. I've plugged it in at least 100 times as

[itex]

v_t = \sqrt{v_1^2 + v_2^2 - 2(v_1)(v_2)cos136}

= \sqrt{26^2 + 21^2 - 2(26)(21)cos136}

=44

= [/itex]

Because that wasn't working I then tried Vector Components and I can't get that to work either. I did:

[itex]

V_x = V_B sin \theta + (-V_A cos \beta )

= 21 sin(22) - 26 cos(22)

=-16

[/itex]

and

[itex]

V_y = V_B cos \theta + (-V_A sin \beta )

= 21 (cos22) + 26(sin22)

= 29

[/itex]

I then tried to figure out

[itex]

\Delta V ^2= \Delta V_x ^2 + \Delta V_y^2

= sqrt{16^2 + 29^2}

= 33

[/itex]

Using that I tried to get the average acceleration by:

[itex]

A_av = \Delta V / \Delta T

A_av = 33 / 2.5x10^-3

A_av = 13.2x10^3

[/itex]

and to find the angle I tried to do :

[itex]

\phi = tan^-1 = 16/29

\phi = 29degrees

[/itex]

However, the answer in my book says that the average acceleration is [itex] 7.3x10^3 [7.5degrees North of West][/itex]

Any help would be amazingly appreciated. Thanks.

A hockey puck rebounds from a board as shown in my diagram. The puck is in contact with the board for 2.5 ms. Determine avg acceleration of the puck over the interval.

V

_{i}= 26 m/s V_{f }= 21 m/sI tried the cosine law but I keep getting 44 m/s and not 18. I don't understand how you guys got 18 m/s. I've plugged it in at least 100 times as

[itex]

v_t = \sqrt{v_1^2 + v_2^2 - 2(v_1)(v_2)cos136}

= \sqrt{26^2 + 21^2 - 2(26)(21)cos136}

=44

= [/itex]

Because that wasn't working I then tried Vector Components and I can't get that to work either. I did:

[itex]

V_x = V_B sin \theta + (-V_A cos \beta )

= 21 sin(22) - 26 cos(22)

=-16

[/itex]

and

[itex]

V_y = V_B cos \theta + (-V_A sin \beta )

= 21 (cos22) + 26(sin22)

= 29

[/itex]

I then tried to figure out

[itex]

\Delta V ^2= \Delta V_x ^2 + \Delta V_y^2

= sqrt{16^2 + 29^2}

= 33

[/itex]

Using that I tried to get the average acceleration by:

[itex]

A_av = \Delta V / \Delta T

A_av = 33 / 2.5x10^-3

A_av = 13.2x10^3

[/itex]

and to find the angle I tried to do :

[itex]

\phi = tan^-1 = 16/29

\phi = 29degrees

[/itex]

However, the answer in my book says that the average acceleration is [itex] 7.3x10^3 [7.5degrees North of West][/itex]

Any help would be amazingly appreciated. Thanks.